In electric furnaces, the slag is at present vigorously aerated so as to improve the performance of the furnace, protect its walls, slow down the wear on the electrodes and reduce the noise. However, unlike the molten slag from blast furnaces, that from electric steel plants cannot be commercially exploited and is simply poured on to slag tips. One of the main reasons for this is that the final conditioning of a foamed slag, for example in the form of an aqueous granulation as carried out on blast-furnace slag, is difficult to execute. Electric-furnace slag in fact forms a voluminous foam that does not flow easily and that is impossible to cool suddenly. In addition, the temperature of an electric-furnace slag is close to that at which solidification takes place (nearly 1600° C.), thus making the slag viscous and also naturally limiting the possibility of any realistic conditioning of the slag.
Another factor limiting the possibilities of commercially exploiting the electric-furnace slag is that it contains free lime. It is therefore impossible to use it as landfill for civil engineering projects since the free lime would cause subsequent swelling of the material. In order to stabilise the slag before use, it is possible to turn over the slag tip and water it regularly, i.e. neutralise the free lime with water, but this is a costly practice and is unreliable.
U.S. Pat. No. 4,179,279 relates to a process which should produce a stabilised electric-furnace slag. According to this process, “red mud”, that is to say a residue of the process of treating bauxite by caustic soda, in the form of briquettes or pellets is charged into molten slag produced by an electric furnace. The physical form of the molten slag is not described. Since the melting point of the red mud is substantially lower than the temperature of the molten slag, up to 20% of red mud can be added to the molten slag without outside heat source, and up to 50% of red mud could even be added to the molten slag if an outside heat source is provided for melting the red mud. According to U.S. Pat. No. 4,179,279, the red mud seems to react with free CaO and 2CaO.SiO2 of the slag. The U.S. patent does not describe the manner in which the molten mixture of slag and red mud could be conditioned for its subsequent commercial exploitation. However, the U.S. patent confirms what was previously said about the rapid cooling of the electric-furnace slag.
It would be advantageous to have available a process that would enlarge the possibilities for commercial exploitation of electric-furnace slag. In conformity with the invention, this objective is attained by a method according to Claim 1.